Support function input to selectClass option.

So you can pass a function into the selectClass option. Pass $el as 'this' to the function to allow context-aware class definitions.

For example:

  $('select').uniform({
    selectClass: function() {
      var cls = this.data('uniform-class');
      return (cls !== undefined && cls.length > 0 ? 'selector ' + cls : 'selector');
    }
  });